As olefin polymerization catalysts, "Kaminsky catalysts" are well known. The Kaminsky catalysts have extremely high polymerization activities, and by the use of them, polymers of narrow molecular weight distribution can be obtained. Transition metal compounds which are known as those employable for the Kaminsky catalysts are, for example, bis(cyclopentadienyl)zirconium dichloride (see: Japanese Patent Laid-Open Publication No. 19309/1083) and ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (see: Japanese Patent Laid-Open Publication No. 130314/1086). It is also known that the olefin polymerization activities or the properties of the resulting polyolefins greatly vary when different transition metal compounds are used in the polymerization. Further, transition metal compounds having a ligand of diimine structure have been recently proposed as novel olefin polymerization catalysts (see: International Patent Publication No. 9623010).
By the way, polyolefins generally have excellent mechanical properties, and therefore they are used in many fields such as fields of various molded products. However, with variation of requirements for the polyolefins, polyolefins of various properties have been desired in recent years. Moreover, increase of productivity has been also desired.
Under such circumstances as mentioned above, there has been desired development of olefin polymerization catalysts having excellent olefin polymerization activities and capable of producing polyolefins of excellent properties.
It is well known that copolymerization of several kinds of .alpha.-olefins and non-conjugated dienes proceeds when Ziegler-Natta polymerization catalysts are used. Since the copolymers thus obtained are useful as rubbers, copolymers of various types have been produced. However, the non-conjugated dienes used in the copolymerization are generally expensive and have low reactivity. Therefore, diene components which are inexpensive and have high reactivity are desired.
Examples of such diene components include conjugated dienes such as 1,3-butadiene and isoprene. Though these conjugated dienes are more inexpensive and have higher reactivity as compared with the conventional non-conjugated dienes, they have problem such that the activities are markedly lowered or only ununiform copolymers of wide composition distribution or wide molecular weight distribution are obtained if the copolymerization is conducted by the use of the conventional Ziegler-Natta polymerization catalysts. In case of a Ziegler-Natta catalyst system using a vanadium compound, the polymerization activities are extremely low, though relatively uniform copolymers are obtainable. In the circumstances, copolymerization of ethylene and butadiene using metallocene catalysts which have been studied extensively and thus known to exhibit high polymerization activities has been investigated (National Publication of International Patent No. 501633/1989).
In the above case, however, it has been reported that from the diene unit and ethylene incorporated into the polyme form together cyclopentane skeleton in the polymer chain, and that the proportion of the cyclopentane skeleton becomes not less than 50% of all the diene units. The conversion of double bonds of the diene unit into the cyclopentane skeleton is very disadvantageous in the procedure of "vulcanizations" required to use the copolymers as rubbers. Further, the cyclopentane skeleton is an unfavorable skeleton because it functions to increase glass transition temperature of the copolymers and is detrimental to the low-temperature properties of the rubbers.
Under these circumstances, as mentioned above, there has been eagerly desired development of copolymers of .alpha.-olefins and conjugated dienes, which have narrow molecular weight distribution and uniform composition and contain almost no cyclopentane skeleton in their polymer chains.